In ionographic devices such as that described by U.S. Pat. No. 4,524,371 to Sheridon et al. or U.S. Pat. No. 4,463,363 to Gundlach et al., an ion producing device generates ions to be directed past a plurality of modulation electrodes to an imaging surface in imagewise configuration. In one class of ionographic devices, ions are produced at a coronode supported within an ion chamber, and a moving fluid stream entrains and carries ions produced at the coronode out of the chamber. At the chamber exit, a plurality of control electrodes or nibs are modulated with a control voltage to selectively control passage of ions through the chamber exit. Ions directed through the chamber exit are deposited on a charge retentive surface in imagewise configuration to form an electrostatic latent image developable by electrostatographic techniques for subsequent transfer to a final substrate. The arrangement produces a high resolution non-contact printing system. Other ionographic devices exist which operate similarly, but do not rely on a moving fluid stream to carry ions to a surface.
One problem affecting the control of image quality in ionographic devices is known as "blooming". Blooming is a phenomenon resulting from the effect of previously deposited ions or charge on the path of subsequent ions directed to the charge retentive surface. The problem is particularly noticeable when printing characters and edges of solid areas, resulting in character defects known as "rocking chair bottoms" (FIG. 1A), "undercutting" (FIG. 1B) and "trapezoids" (FIG. 1C), (with input bit maps shown in dashed lines).
U.S. Pat. No. 4,593,994 to Tamura et al. discloses an ion flow modulator for use in a photocopying machine, including a common electrode formed on one major surface of an insulating substrate and a plurality of ion control electrodes formed on the other major surface of the insulating substrate. U.S. Pat. No. 4,562,447 to Tarumi et al. discloses an ion modulating electrode for a recording unit of an electrostatic recording apparatus including one row of apertures capable of enhancing or blocking a passage of ion flow. U.S. patent application Ser. No. 370,317, entitled "METHOD AND APPARATUS FOR CONTROLLING ION TRAJECTORY PERTURBATIONS IN IONOGRAPHIC DEVICES", filed 6/22/89, and assigned to the same assignee as the present application, teaches the use of an electrode extending across the array of modulation electrodes with single bias voltage applied thereto.